The invention relates to digital communications, and, more particularly, to discrete multitone communication systems and corresponding circuitry and methods.
Digital subscriber line (DSL) technologies provide potentially large bandwidth (e.g., greater than 20 MHz for subscribers close to the central office) for digital communication over existing telephone subscriber lines (the copper plant). The subscriber lines can provide this bandwidth despite their original design for voice band (0-4 kHz) analog communication. In particular, ADSL (asymmetric DSL) adapts to the characteristics of the subscriber line by using a discrete multitone (DMT) line code with the number of bits per tone (subchannel) adjusted to channel conditions. The bits of a codeword are allocated among the subchannels for modulation to form an ADSL symbol for transmission. FIG. 2a illustrates the use of the Fast Fourier transform in a system having, for example, 256 tones with each tone treated as a QAM subchannel (except dc tone 0) and so the kth tone encoding corresponds with a complex number X(k) for 0≦k≦255. Extending to 512 tones by conjugate symmetry allows the 512-point IFFT to yield real samples x(n), 0≦n≦511, of the transformed block (symbol); and a DAC converts these samples into a segment of the transmitted waveform x(t). FIG. 2a also notes a cyclic prefix for each symbol which allows for simplified equalization for the interference of successive symbols which arises from the non-ideal impulse response of the transmission channel.
For example, Annex A of the ADSL standard G.992.3 has subchannels separated by 4.3125 KHz and a band extending up to 1104 KHz for 256 subchannels. Annex A also provides power spectral density (PSD) masks for both central office and customer transmitters plus 0-40 dB power cutback levels to be applied according to dynamic range measured by the central office and customer receivers during initialization procedures. In higher bandwidth versions of the ADSL standard, the PSD can be shaped for efficient operation, such as limiting crosstalk among physically adjacent subscriber lines and reducing the transmit power requirements. However, power cutbacks for transmitters with shaped power spectral densities (PSDs) can render the low power portions of the spectrum useless.